This invention relates to semiconductor lasers and, in particular, to buried heterostructure semiconductor lasers utilizing Impurity Induced Disordering (IID).
Recently, advances have been made in the art to better delineate the bandgap and refractive indices properties in a single semiconductor device by impurity induced disordering (IID) where zinc is selectively diffused into the multiquantum well structure thereby disordering quantum well structures epitaxially deposited as part of a semiconductor device. An example of IID is U.S. Pat. No. 4,378,255 to Holonyak wherein there is taught the technique of selectively disordering a multiple quantum well structure or superlattice in a semiconductor device through the employment of a zinc diffusion causing an upward shifting of the bandgap of the disordered material compared to regions of the multiple quantum well structure where disordering has not occurred. Such diffusions can be generally carried out in a temperature range of 500.degree. C.-600.degree. C., which is lower than the epigrowth temperature of III-V materials, which is about 750.degree. C.
Such disordering is also possible with other elements such as Si, Ge and Sn but at higher temperatures, e.g., about 675.degree. C.
Also, disordering is possible through implantation of elements acting as shallow or deep level impurities, such as, Se, Mg, Sn, O, S, Be, Te, Si, Mn, Zn, Cd, Ln, Cr or Kr followed by a high temperature anneal best performed in an As environment in the case of III-V epigrowth to prevent the outdiffusion of As from the growth. In the case of impurity implant followed by an anneal, the anneal temperatures are relatively at higher temperatures compared to diffusion temperatures, e.g., above 800.degree. C.
Teachings relative to smearing or disordering of multiquantum wells in heterostructure lasers lowering the refractive index in disordered regions can be found in the following references: W. D. Laidig et al, "Disorder of an AlAs-GaAs Superlattice by Impurity Diffusions", Applied Physics Letters, Vol. 38(1), pp. 776-778, (May 15, 1981); W. D. Laidig et al, "Induced Disorder of AlAs-AlGaAs-GaAs Quantum Well Heterostructures", Journal of Electronic Materials, Vol. 11(1), pp. 1-20 (1982); J. P. Leburton et al, "Index of Refraction of AlAs-GaAs Superlattices", Journal of Applied Physics, Vol. 54(7), pp. 4230-4231 (July, 1983).
Recently, there has been proposed a buried heterostructure semiconductor laser of the GaAs/GaAlAs regime and referred to as transverse mode controlled laser having a buried multiquantum well (BMQW). In order to bury the multiquantum well active region, impurity induced disordering (IID) where zinc is selectively diffused into the multiquantum well structure comprising the active region. This diffusion through the active region adjacent both sides of a mask protected region formed a buried multiquantum well active region surrounded on adjacent sides by a zinc diffused region which smeared out the multiquantum well integrity of the regions of the active layer. Zinc diffusion at a relatively low temperature promotes the intermixing of Al and Ga in a GaAs/GaAlAs superlattice making up the active region which results in the formation at these diffused active regions of a GaAlAs alloy with an averaged AlAs mole fraction. Due to the disordering, the buried multiquantum well active region has a refractive index larger than the adjacent averaged GaAlAs alloy regions so that this refractive index difference enables the formation of an optical waveguide. The disordered regions provide a barrier for confining the injected electrons and holes to the active region. The benefits can be realized by a single diffusion step, eliminating the two step fabricating process, for example, used in the past to fabricate buried heterostructure lasers. The basis for the above-outlined proposal can be found in the article of T. Fukuzawa et al entitled "GaAlAs Buried Multiquantum Well Lasers Fabricated by Diffusion Induced Disordering", Applied Physics Letters, Vol. 45(1), pp. 1-3, (July 1, 1984).
There are, however, certain drawbacks to the buried heterostructure proposed by Fukuzawa et al. The disordering of the multiquantum wells requires diffusion across or through the laser active regions to be disordered thereby creating problems relative to free carrier absorption in the optical cavity of the multiquantum well active region and, further, the resultant refractive index changes in diffused regions of the active region may be too large for stable single mode operation up to desired power levels.